I have been a little bit frustrated since I discovered my brand new 580EX II unit didn't have an Optical Slave mode so I started to search for an solution that wouldn't cost me half of the price of the unit itself to work around this issue.
I ended up finding some pretty cheap eBay remote RF triggers that most of you already know and I decided to give them a try even reading about all the reliability problems this kind of cheap solution has.

I ended up buying an BlaZZeo MegTrig RF Speedlites Trigger (re-branded Cacuts V2s) for about 20 USD including shipping to Brazil. At that price, you just can't beat them, but how would it work with my 580EX II baby?
Well, let's see...

This is an 4 channel trigger, what means you can set up to 4 different frequencies so you won't interfere with your photographer friends, firing their flashes and vice versa.
That's an real good thing over the dumb Optical Slave, where anyone with an point and shoot camera can fire your flash and leaving you without light for your own camera. But anyways I still think it's an crime the 580EX II lacks this option being such an expensive unit. Well....
To adjust the frequency you just set up the small blue/white keys on both, the receiver and transmitter, to the same combination and you're good to go.

Installation is very forward and takes only a few seconds.
You plug the Transmitter on your camera's hot shoe, then you plug your flash unit on your Receiver, set up the frequency, turn everything on and you can start shooting.
The Receiver uses 2x AAA batteries (came with) while the Transmitter won't take any batteries. I suspect it has an internal battery since you can use it out of the camera without any apparent power source.

The build quality is pretty low, but for 20 bucks I think it's OK. It's not weather sealed and won't last forever. If you want better build quality and more longevity, you can go for either the Canon ST-E2 (IR ETTL compatible for around 200 bucks) or Pocket Wizards (RF ETTL compatible for around 200 bucks each receiver and transmitter).

The Receiver has an dumb hot shoe so you can plug it to light stands. It has an thread on it so you can plug it to regular tripods without adapters. The only thing is that this L Shaped support is way too fragile and the flash will shake at even the softer touch. There are some work around solutions for it on Youtube but I just left it the way it came.

Since it has an "one fits all" design, it won't attach perfectly to the 580EX II weather sealing design. The lock can't slide all the way, staying a bit over the middle, so you gotta take care when moving your flash unities. This picture illustrates it better (take an look at the Lock slider on the 580EX II):

The Transmitter installation is more solid . You just have to plug it on the camera's hot shoe and screw. It holds on pretty tight and won't fall.
On the transmitter there's an Flash Preview Button, so you can fire your flash pressing it to see shadow casting and other adjustments you may want to do:

Performance and RELIABILITY

Most of concerns about this kind of unit is about its reliability. Can you trust the remote trigger? Can you be confident it will fire when needed?
Well, most of tests I've been reading and watching on Youtube shows it is pretty unreliable, so I decided to test it myself.
On Youtube tests revealed this kind of solution to have at least 30% FAILURE , what's pretty bad.
My test:I mounted my flash on the doorway and started walking with the camera/transmitter until the signal was lost. Then I walked back 1 step and shot 100 frames checking for how many would not fire.
At my first test, I could walk 20 feet, step 1 back and around 15 flashes didn't fire, so reliability was deeply compromised. But then I got an idea... I CHANGED THE BATTERIES that came with the trigger (no brand Chinese batteries) and replaced by Duracell alkalines.
In two words??
WAAAAAAAAAAAAAAAAAAY BETTER

I could walk 32 feet on the door way until I lost the signal. Stepped 1 back and fired 100 shots.
ALL THE 100 SHOTS FIREDSo I put the camera on my back becoming in the middle of the signal, and all the 100 shots fired again. Then I closed the glass door and again put the camera behind me and guess what? All the 100 shots fired again.
Around 31 feet for 20 bucks? That's less then 1 buck per feet! Awesome!

Shooting on Burst mode revealed it can easily keep up the 3FPS of the Slow Burst in the 40D, but CANNOT keep up with the 6.5 FPS on the High Burst. On that mode it will fire 2 flashes every 3 shots, making one totally dark and unusable.

CONCLUSION

I am pretty happy with this thing. It's lightweight, it's cheap and, specially, it solves my problem.
According to my tests the reliability problems seem to be much more an battery problem than an communication problem. Maybe I just got lucky... who knows?

But bottom line here is:
If you want professional rock solid communication and can't miss one shot, go for the Pocket Wizards. Hands down. It's a lot more expensive but will save your day.
On the other hand, if you are an hobbyist and won't mind the risk of losing one photo once in a while (I didn't lose any up to 30 feet), this may be an good solution.

I have the same system and I am tying to use it with my Canon Rebel xsi. Although it "tests" well by hitting the test button on the transmitter, I cannot get the camera to recognize it and activate it from the hot shoe - am I missing an obvious camera setting?
Along those lines - there is a small plug jack on the side of the transmitter - does anyone know what sort of plug fits in that?
Thanks!

I have the same system and I am tying to use it with my Canon Rebel xsi. Although it "tests" well by hitting the test button on the transmitter, I cannot get the camera to recognize it and activate it from the hot shoe - am I missing an obvious camera setting?Along those lines - there is a small plug jack on the side of the transmitter - does anyone know what sort of plug fits in that?Thanks!

There is a little LED on the top of the transmitter. Does it blink when you press the Test button? It may be an defective unit, since no camera setting should act over this little button.
You can try to remove the transmitter from your camera and press the button again. The LED should blink and the flash fire even if it's not connected to the camera.

The test button and LED work fine, on and off the camera and with the camera powered on or off. It fires both the receivers every time.
What it does not do is actually activate when I press the shutter button, that is, try to take a picture.
I'm an experienced photographer with old school equipment, but new to the automatic cameras.
Did you have to make any changes in the Flash settings in the camera menu? How do you have your camera set when you shoot?
I feel like I am missing something obvious here.

Oh, now I got it. Sorry... I totally misunderstand what you said before.
Well.. just to be sure, go to your Menu -> Flash Control and see if the "Flash Firing" is enabled. Maybe you can try to "Clear external Flash settings".
I'm using the 40D, but I'm sure the XSi has those options too.

i hope you don't mind me asking in this topic but... i'm going to buy this and i have a question; if i buy the canon 430ex i can surely use it on these triggers and on the camera itself - use all manual functions BUT if i buy the 420ex, meaning the one without manual functions on the flash itself how will it perform on these triggers? will it just flash on 100% all the time? or will i be able to control it through the camera via radio waves? i don't want to spend a lot of money on the 430ex if i could do perfectly well with the 420ex. thanks for any answers.

When you're using the budget wireless trigger, your only option is "everything in manual". Flash on, or Flash off. So, I believe it will be 100% all the time. The only control is basically your camera adjustments. I'm using a couple old Vivitar 283's. I intend to eventually get a 430EX to use as a main flash, and might use it in conjunction with the slave flashes.
You might consider a full hot shoe sync cord if you are using a 420EX.

i thought about the vivitar's but i'd also like to use a flash on my camera at times and i hear there's a risk at doing so... i'd really like to hear from someone who has a 420ex or the sigma 500-530 dg st which also doesn't have full manual controls - how it works with these triggers...